1. Field of the Invention
The present invention relates to signaling for enhanced 911 systems, and more specifically, to testing such signaling over digital loop carrier arrangements.
2. Description of the Related Art
Enhanced 911 (E911) service refers to establishing communication between customer telephones and the nearest Public Safety Answering Point (PSAP), which is an answering location for 911 calls originating in a given geographic area.
The operation of this E911 service will now be described with reference to the prior art E911 system 100 shown in FIG. 1. All communication paths in this system 100 are conventional two-wire paths, which use a xe2x80x9ctipxe2x80x9d wire and a xe2x80x9cringxe2x80x9d wire to define a loop (also known as a channel) used for communication. In this system, one of the customer telephones 110 goes xe2x80x9coff-hookxe2x80x9d to report an emergency by dialing 911. In response to the dialed digits, the associated Private Branch Exchange (PBX) 120 senses this off-hook condition, which is a low resistance tip-to-ring path. The PBX 120 is typically large, with many telephones 110 widely dispersed, to merit the deployment of a dedicated analog E911 trunk 155. Such an E911 trunk is typically dedicated solely for E911 service, and is configured for outward (i.e., PBX-originated) communication. After sensing the off-hook condition, the PBX 120 seizes the trunk 155 (i.e., wires between the PBX 120 and the local central office (CO) 160) by closing the loop defined by the tip/ring pair of wires in the trunk 155. A trunk seizure condition in E911 signaling is defined as a call state initiated by the PBX 120 in response to a customer telephone 110 in which the CO 160 prepares to receive signals.
After the CO 160 acknowledges seizure of the trunk 155 by reversing the battery polarity applied to the trunk 155, the PBX 120 sends two separate data messages: one identifying the E911 switch (170) to handle the call during the emergency, and the second identifying the telephone 110. An E911 switch 170 may be a central office switch which has been programmed to handle E911 signaling and switching. As used herein, the term xe2x80x9cE911 switchxe2x80x9d refers to either the above-described E911 CO simulator, or a dedicated E911 switching unit. When the E911 switch 170 receives the messages identifying the closest PSAP 130 and the calling telephone 110, the E911 switch 170 makes a connection to the closest PSAP 130. In some implementations a separate box 150 interfaces the PBX 120 to the trunk 155, to provide E911 compatibility to a legacy PBX. The data messages between the PBX 120 and the CO 160 are sent using a multi-frequency (MF) protocol, which uses pairs of tones with frequencies contained within the voice bandwidth of the respective channel units (not shown, but defined as those units which control communication along one or more communication channels) in and between the PBX 120 and CO 160. Thus, the DC signaling characteristics of the channel units operating the PBX/CO link are used only for supervision in initiating the E911 call and in terminating it.
The above-described E911 service, where the PBX 120 seizes the trunk 155 by closing the loop and the CO 160 terminates the trunk 155 by applying reverse battery polarity to acknowledge, is used on a conventional analog trunk 155. Such signaling between the PBX 120 and CO 160 is generically termed xe2x80x9cloop reverse batteryxe2x80x9d (LRB) signaling. This E911 interface at the PBX end is specified by an American National Standards Institute (ANSI) standard, ANSI T1.411-1995, xe2x80x9cInterface between Carriers and Customer Installationxe2x80x94Analog Voicegrade Enhanced 911 Switched Access Using Network-Provided Reverse-Battery Signaling.xe2x80x9d
Recently, so-called digital loop carrier (DLC) systems have been developed and implemented, where a larger number of channels may be implemented on fewer wires than in a conventional analog network. DLC channel units accomplish this greater channel density by time division multiplexing digital data for a number of channels onto two pairs of wires. For example, 24 communication channels may be implemented on two wire pairs in a DLC system, whereas the two wires only provide one channel in an analog implementation.
As shown in FIG. 2, DLC channel units 210, 220 are typically located in a central office terminal (COT) 250 in the CO 160, and in a remote terminal (RT) 200 near the PBX 120 to coordinate signaling therebetween. DLC trunks 230 and 240 are able to be seized only by an originating channel unit 210, so in a seizure sense, the trunks arc unidirectional. However, once communication has been established between the CO and PBX, two-way traffic occurs over the DLC trunk. RT 200 includes an originating channel unit 210 connected to DLC trunk 230, which is terminated by terminating channel unit 220 in COT 250. Originating channel unit 210 in the RT 200 seizes the trunk 230 when a PBX originated call occurs. Similarly, DLC trunk 240 is seized by channel unit 210 in the COT 250
Telcordia (previously named Bellcore) has published three standards for DLC systems that specify signaling arrangements between a central office (CO) and a Private Branch Exchange (PBX) for direct-inward-dialing (DID) service (i.e., calls originating from the central office) using the loop reverse battery (LRB) signaling protocol. These standards are termed TR-08, TR-57, and GR-303. TR-08 and GR-303 cover xe2x80x9cintegratedxe2x80x9d DLC systems (i.e., a digital facility terminated directly by a digital interface of a switch used to connect one channel to another in the central office). TR-57 specifies requirements for a xe2x80x9cuniversalxe2x80x9d DLC that uses a central office terminal (COT) 250 to convert the digital signal from the DLC 230 to an analog signal which is sensed by an analog switch 260 in the CO. In conventional DID service, the CO 160 seizes the DLC trunk 240, and the PBX 120 acknowledges such seizure via a RT 200 which contains a DLC channel unit 220. A trunk seizure condition conventional DID service is defined as a call state initiated by the CO in which the PBX prepares to receive incoming signals.
Where the telephone company employs a universal DLC system to assist in connecting the CO switch and the PBX over the loop pair, the channel units of the DLC system at the remote terminal (RT) location present the same interface to the PBX as do the channel units of a conventional analog CO. Similarly, the COT channel units present the same interface to the switch in the CO as do the channel units of a conventional analog PBX. As mentioned above, the DLC systems in use today seize the trunk at the CO to implement DID. However, E911 service mandates seizing the trunk at the PBX, and such trunk seizure is not specified by any of the existing TR-08. TR-57, or GR-303 digital loop carrier standards.
Accordingly, a manner of implementing enhanced 911 service on digital loop carrier systems is needed. Further, the above-described conventional analog enhanced 911 systems are difficult to test prior to deployment, and, once they are deployed, each channel must be tested individually. Thus, a method of testing enhanced 911 service on digital loop carrier systems prior to deployment is also needed.
The present invention provides a method and system for rapidly and automatically testing E911 systems using digital loop carrier trunks in a laboratory prior to deployment. A testbed running a testing program evaluates the functionality of a digital loop carrier trunk, a channel unit pair, and an E911 switch by measuring a delay and duration of an acknowledgement pulse from the E911 switch in response to an off-hook condition at a simulated PBX. The testbed and testing program also evaluate the functionality of the E911 switch and a simulated PSAP by measuring a delay and duration of a ring signal from the E911 switch in response to an emergency signal sent by the simulated PBX. Once connection between the simulated PBX and the simulated PSAP has been established, the end-to-end signal loss is also measured.